1. Field of the Invention
The present invention relates to automotive air blowers and particularly to automotive superchargers. More specifically, the present invention is concerned with automotive air blowers of the type disclosed in WO 2004/072449.
2. Description of the Related Art
WO 2004/072449 discloses an automotive supercharger comprising an air impeller, which is situated, in use, in the inlet duct of an engine and is connected to the output shaft of a transmission system, the input shaft of which is connected, in use, to be driven by the engine. The transmission system comprises a three-branch epicyclic gearset. In the embodiment of FIG. 1, the gearset comprises a sun wheel, which is connected to the output shaft of the gearset, constituting a first branch, and is in engagement with a plurality of planet wheels, which are rotatably carried by a rotatable planet carrier, constituting a second branch and connected to the input shaft of the transmission system, and are in engagement with an annulus wheel, constituting the third branch. The second and third branches are connected to the rotors of respective electrical machines constituting motor/generators, the electrical stator connections of which are connected via a controller arranged to control the flow of power between them.
In use, mechanical power is transmitted from the engine to the impeller via the transmission system. One or other electrical machine is usually caused to operate as a generator and the power it produces is transmitted to the other motor/generator, which operates as a motor. The speed of the impeller can thus be controlled as desired independently of engine speed to match the instantaneous supercharging requirement of the engine.
The prior document specifically states that one or other of the motor/generators may be used for engine starting purposes and this is clearly advantageous because it will permit the conventional automotive starter motor to be omitted. However, if one were to attempt to use the motor/generator connected to the planet carrier as a starter motor in the construction of the prior application, it would in fact be wholly ineffectual because rotating the planet carrier when the input shaft is stationary will merely result in rotation of the transmission output shaft and thus of the air impeller and not in the rotation of the transmission input shaft and thus of the engine itself, which is of course necessary if the engine is to be started.
In practice, the electrical machine connected to the transmission shaft runs at relatively high speed whilst that connected to the planet carrier may have a lower maximum speed requirement. In order to be able to start an internal combustion engine, an electrical starter motor must be able to generate a high torque level, at least for a brief period of time. This means that if the electrical machine connected to the transmission input shaft is to be used for starting purposes, it will have to be of high speed/high torque type. However, an electrical machine of high speed/high torque type is expensive, bulky and heavy and this is commercially unacceptable for most applications.